1. Field of the Invention
The present invention relates to a catalyst for the exhaust gas purification, a method for the production thereof, and a method for the purification of an exhaust gas using such a catalyst. More specifically, the present invention relates to a catalyst for the exhaust gas purification having excellent ignition performance and exhaust gas purification performance, in particular, NOx purification performance, a method for the production thereof, and a method for the purification of an exhaust gas using such a catalyst.
2. Description of the Related Art
To respond to variation of exhaust gas atmosphere from an engine, a three way catalyst comprising palladium, rhodium and platinum in combination has been heretofore used to efficiently purify CO (carbon monoxide), HC (hydrocarbon) and NOx (nitrogen oxide).
Also, the addition of an alkaline earth metal to the three way catalyst has been known to enhance catalytic action of palladium. Therefore, for example, JP-A-9-57066 discloses a catalyst for the exhaust gas purification which is formed by providing on a honeycomb-like monolithic carrier a first catalyst layer containing a refractory inorganic substance on which one or more members of precious metals selected from the group consisting of platinum (Pt), palladium (Pd) and rhodium (Rh) are supported, and further providing on the first catalyst layer a second catalyst layer containing an inorganic substance mainly composed of a porous crystalline aluminosilicate on which a copper (Cu) component is supported, characterized in incorporating a barium (Ba) component in the first catalyst layer. In addition, JP-A-10-52640 discloses a catalyst for the exhaust gas purification, characterized in covering a monolithic structure with a catalytically active component containing (a) a palladium and an alkaline earth metal oxide, (b) a zirconium oxide having lanthanum and cerium supported thereon, and (c) a refractory inorganic oxide. US-A-2001-31699 discloses a catalyst for the exhaust gas purification which comprises a cylinder-like carrier having a lot of through holes, each of the holes penetrating in an axis direction; a supporting layer of a refractory inorganic oxide formed at the inner surface which partitions the through holes; and a catalyst component of a precious metal supported on the supporting layer, and composed of an upstream side catalyst arranged at the upstream side relative to exhaust gas flow and a downstream side catalyst arranged at the downstream side relative to exhaust gas flow, characterized in that the upstream side catalyst contains one member selected among palladium, palladium and rhodium, or palladium and platinum as the precious metal; the supporting layer contains at least barium, and alumina containing lanthanum; the downstream side catalyst contains at least one member selected among platinum, palladium and rhodium as the precious metal; and the supporting layer contains at least one member selected among alumina containing lanthanum, a solid solution of cerium, or cerium and zirconium, and a solid solution of cerium and zirconium and yttrium.
As described above, although an alkaline earth metal is useful to palladium, it may be catalytic poison, in particular, to rhodium. Accordingly, co-presence of an alkaline earth metal with rhodium, as described in JP-A-9-57066 and US-A-2001-31699, incurs a problem of lowering NOx purification capacity by rhodium, or lowering oxidation performance.
On the other hand, although rhodium has very high NOx purification capacity, its amount used in catalyst components should be limited, due to being very expensive. Accordingly, in a catalyst for the exhaust gas purification as described in JP-A-10-52640, because rhodium is not used, further enhancement of NOx purification capacity must be required or has a problem of lowering oxidation performance.
Therefore, presence of rhodium is essential as a component of a catalyst for the exhaust gas purification, and maximal utilization of catalytic action thereof is required.